In the field of minimally invasive microsurgery and therapy, in-vivo feedback information about the contact force exerted at the tip of instruments or tools is an important parameter required by surgeons to improve the outcome of their interventions. Frequently, the reduced access conditions affect the feel of interaction forces between the instruments and the tissues or organs being treated, and the forces involved are below the human perceptual thresholds.
A particular case of application of this invention relates to palpation procedures during middle-ear surgery, to evaluate the mobility of the ossicular chain during tympanoplasty. The knowledge about the ossicular mobility is important for decisions regarding surgical procedures as well as for the prognosis of improvement of hearing level. For example, typical contact forces on microsurgical tools for the evaluation of the mobility of the stapes bone are less that 10 mN, below the threshold of the operator's tactile sensitivity.
No such 3-dimensional force sensing tool for middle-ear is known. The only instrument reported is a tool for the evaluation of the stapes bone which measures only in one axial direction, as described in “An apparatus for diagnostics of ossicular chain mobility in humans” by Takuji Koike et al, International Journal of Audiology 2006; 45:121-128.
Another particular case of application of the invention relates to micro-force sensing tools for retinal microsurgery. Retinal microsurgery requires delicate manipulation of retinal tissues, and tool-to-tissue contact forces are frequently below human perceptual thresholds. Typical contact forces on microsurgical instrument tips during retinal surgery are less that 7.5 mN.
No such 3-dimensional force sensing tool for retinal surgery is known. An instrument is reported in which micro-forces in two lateral directions are measured using fibre Bragg grating techniques in “A sub-millimetri, 0.25 mN resolution fully integrated fibre-optic force sensing tool for retinal microsurgery” by Iulian Iordachita et al, published online 15 Apr. 2009 by International Journal of Computer Assisted Radiology and Surgery (Int J CARS).
It therefore would be desirable to provide a method for detecting and monitoring three-dimensional contact forces between the tip of a microsurgical instrument or tool and the tissue or organs to be explored and treated.
Known systems for catheter applications are reported to measure the contact forces at the tip of the catheter using fibre optical measuring techniques, where the force sensing elements are located close to the tip.
Force sensing elements for such microsurgical instruments used for catheter applications comprising monolithic cylinder structures are well known. By applying a three dimensional force on the tip of the structure, the structure is deformed in a predefined way given by a set of notches in the structure. These notches define several elastic zones making the structure flexible in some of the directions x, y and/or z. Optical fibres integrated in the structure admit to determine a displacement of individual parts of the structure, which is proportional to the force applied to the tip. The fibres enter the structure from the bottom of the structure and are guided in channels ending at one of the notches of the structure. Light emitted from the fibre is retro-reflected by a surface on the structure opposite of the fibre end. It re-enters the optical fibre and is evaluated to determine the distance of the fibre end to the surface on which the light was reflected.
Previously-know systems to measure contact forces at the tip of a catheter are described in US 20080009750. It relates to a monolithic structure which is very complicated in design and requires a rotation of the structure during manufacturing. Further, the structure is a tube with a wall thickness of 0.5 mm and a total diameter of 5 mm. It is not possible to reduce the size of such a structure for reasons of manufacturing and mechanical stability.
In US20090177095 a similar tube structure with three identical notches is shown, spaced apart from each other along the central axis. Each notch is made by a cut from one side which is in 120° rotation from the other two sides. In each of the three notches one optical fibre ends to measure the distance to the structure opposite the fibre end. It is inevitable that some of the fibres cross other notches until they reach their destination. The fixation of the fibres is very demanding. Although this structure must not be rotated during manufacturing of each single notch as the one mentioned before, it still must be rotated in-between cutting the notches. It is still very expensive for manufacturing in the required precision. Further, the structure is stiff in the axial direction and flexible in the radial directions, so it is suited only to detect forces applied on a tip close to the structure.
A further structure is given in WO2009114955. This structure is made by cutting in a plane defining blades in any of the planes x-y, x-z or y-z, defining the required flexible zones. Again, three optical fibres determine the distances of the gaps at the notches to determine the forces applied. This structure can be made smaller having a diameter less than 2.5 mm, between 1.7 and 2 mm. Unfortunately, the structure is very large in z-direction and complicated, containing 7 notches. Further versions shown in the same application are smaller in length and comprise only two or four cuts, but they are designed to be very flexible in the direction perpendicular to the axis.
These structures are simple for manufacturing but can not be used for the purpose indicated above, namely for middle-ear surgery or retinal microsurgery. Such tools contain a long and thin shaft at the front of the sensing structure with a tip, whereas a force applied at the distant tip in a direction perpendicular to the z-axis (in direction x, y) has an effect to the structure which is 5 to 20 times stronger than a force applied in the same direction but close to the structure due to the moment of inertia.
Further, it is more and more common that medical instruments become disposable for reducing the risk of infections. Therefore, the structure must be as simple as possible to be manufactured at low cost.
All structures known are used with catheters and therefore designed for lateral forces Fx, Fy and an axial force Fz, whereas the lateral forces are applied on the top of the structure close to the flexible area, not more distant to its flexible area than three times the diameter d of the structure.
These structures can not be used in surgery, where lateral forces are applied distant to the flexible zone of the structure, where the distance D is up to 20 times the diameter d of the structure. It has been shown that the known structures can not be adapted for this application by re-dimensioning of the structure.